As pretty and intricately constructed as an 18th-century corset, The Duchess is a slightly too serious period costume drama based on Amanda Foreman's best-selling biography Georgiana, Duchessof Devonshire.

In 1774, Georgiana Spencer, an English

aristocrat (and ancestor of Lady Diana Spencer), married William Cavendish, the Duke of Devonshire and one of the richest and most powerful men in England. Georgiana was a celebrated and scandalous figure, a fashionable socialite who was active in Whig politics and cultivated a considerable cult of personality - we are supposed to notice the echoes of her life in the 20th-century tragedy of Princess Di.

Chief among the correspondent themes of the lives of these Spencers is the idea that both of them were married off for political expedience - Georgiana's loveless match with Cavendish was a subject of much gossip at the time, inspiring Richard Brinsley Sheridan's 1777 comedy of manners School for Scandal.

That's the hook, and director Saul Dibb has the

benefit of a pair of beautifully subtle performances

by Keira Knightley as Georgiana (or, as the duke calls her, G) and Ralph Fiennes as her stolid, passive-aggressive (but not exactly evil) husband.

Immensely popular with the common folk, G throws herself into her political work - and an affair with Earl Charles Grey (DominicCooper) an ambitious candidate for prime minister - while her husband takes her best friend Bess (Hayley Atwell) as a very public mistress.

While the film only occasionally rises above standard tasteful costume-melodrama levels, G's fashion-icon status allows Knightley dozens of frock changes while the film glides lightly over its secondary theme of celebrity impingement upon politics.

Similarly, Fiennes' restrainedportrayal of the duke as a plodder without malice, matched with a very modern type of highmaintenance woman, is a marvel of nuanced, measured acting. Fiennes probably won't get the sort of attention that draws awards - the film is too standard, his role is unflashy - but he does more with flustered seething than most actors do with pages of monologue.